Group Report: Rheology of Fault Rocks and Their Surroundings

Terry E. Tullis, Roland Bürgmann, Massimo Cocco, Greg H. Hirth, Geoffrey C. King, Onno Oncken, Kenshiro Otsuki, James R. Rice, Allan M. Rubin, Paul Segall, Shawn Shapiro, & Christopher A. Wibberley

Published 2007, SCEC Contribution #1392

At depth, all major faults undergo aseismic motion, and some faults slip aseismically at shallower levels. However, most slip on shallow faults occurs seismically, generating earthquakes. Due to the importance of earthquake-related hazards to society and because more remains to be understood about the mechanics of earthquakes than about stable motion, the focus of this report is on understanding the processes involved in seismic motions. In addition, we consider the process by which stress accrues on faults, as well as the interactions between seismic events and the aseismically deforming crust at depth. We organize the discussion around the seismic cycle, the repeated occurrence of earthquakes, and its stages: prior to an earthquake, slip increases quasistatically during a nucleation phase. The coseismic stage, i.e, when the earthquake occurs and nearly all slip accumulates, coincides with dynamic rupture and is relatively short (seconds to minutes). It is followed by post- and interseismic phases, characterized by gradually decreasing slip and healing of the slip surface, until the next earthquake occurs. Coseismic slip increases stress below the maximum depth of rupture, accelerating aseismic deformation during the postseismic period and reloading seismogenic portions of the fault (Figures 6.1a, b, this volume). During much of the earthquake cycle, strain in the vicinity of active faults accumulates linearly in time, reflecting steady loading by tectonic plate motion. Typically, successive earthquakes differ from one another in their location, nucleation, rupture, and postseismic creep, so that although the concept of an earthquake cycle is useful, it should not be interpreted as a regular series of identical events.

Tullis, T. E., Bürgmann, R., Cocco, M., Hirth, G. H., King, G. C., Oncken, O., Otsuki, K., Rice, J. R., Rubin, A. M., Segall, P., Shapiro, S., & Wibberley, C. A. (2007). Group Report: Rheology of Fault Rocks and Their Surroundings. In Tullis, T. E., Bürgmann, R., & Cocco, M. (Eds.), Tectonic Faults: Agents of Change on a Dynamic Earth, (, pp. 183-204) Cambridge, USA: MIT Press